A thin-film transistor (hereinafter also referred to as “TFT”) is widely used in various kinds of flat-panel display devices such as a liquid crystal display device and an organic electroluminescence display device.
Recently, attention has been paid to an oxide semiconductor as a material which is applicable to channel layers of TFTs. For example, TFTs using an In—Ga—Zn—O (hereinafter “IGZO”)-based amorphous oxide as the material of semiconductor layers have vigorously been developed. In many cases, a TFT including an oxide semiconductor thin film of, e.g. IGZO adopts an inverted staggered structure, that is, a bottom gate structure, which is easy to fabricate.
However, for example, when the TFT including the oxide semiconductor thin film is applied to the display device, there are two problems with the inverted staggered type. One problem is that the channel length cannot be decreased because of the structure of the TFT, and it is difficult to decrease the circuit area and to enhance the capability (or to improve the ON current). The other problem of the inverted staggered type is that protection of the back channel side is necessary, and if such protection is not provided, the amount of oxygen in the film of the oxide semiconductor of the channel portion would vary in the process after the TFT formation, leading to instability of transistor characteristics. Specifically, the channel portion becomes a resistor, and such problems will occur that the TFT fails to function as s switching element and the threshold voltage of the TFT greatly varies.